The present invention relates to an electroacoustic mechanism for the production of low frequency sound in the acoustic spaces typical of the passenger compartments of vehicles. Although the technique described herein is expected to be used principally for high-fidelity music reproduction, it also is particularly attractive for applications in active noise cancellation, and may allow significant weight savings over conventional sound deadening. An actual device constructed according to these teachings has been shown to exhibit in-vehicle pressure response flat to 11 Hz, with extremely high displacement power handling in the pass band with the enclosure volume being one-half cubic foot.
The production of low frequency sound in domestic room sized acoustic spaces presents physical problems that are well understood. The solution to these problems all ultimately involve moving large amounts of air, i.e. in acoustics referred to as volume velocities of air.
The particular acoustic characteristics of vehicular passenger spaces is less generally understood. Typical existing practice has largely been a simple transference of techniques and actual devices used in domestic, large room applications, with little, if any, regard for the unique conditions and requirements of vehicular passenger compartments. In the development of the present invention, the analysis, simulation, and actual measurement of these spaces has yielded a new and optimal solution for the production of sound in this specific environment.
In adapting home sound equipment for vehicle use, modifications have typically been limited to improvements in mounting, or the substitution of plastic cones for paper in order to improve moisture resistance. Enhancements for a desired frequency response have largely been in the form of electronic equalization prior to amplification. Although this technique affords great flexibility in "tuning" a given system, it necessarily limits the maximum power handling over the part of the spectrum which has been boosted. This can become a serious problem at low frequencies where large driver excursions may be required to achieve a desired response.
Acoustical considerations have largely been based on a system's free field behavior, where the transducer may be considered a point source. In such an environment, flat acoustic pressure response is achieved by driving the air load under a constant acceleration regime.
In automotive applications, the typical practice has been to operate the system below resonance, where the driver operates in a constant displacement regime. Such systems, at best, exhibit a rolloff asymptotically approaching 12 dB./octave under free field conditions, which roughly compliments the rising response typical of vehicular interiors, as described below. The chief limitation of these systems lies in the large excursions required to produce a given volume velocity. Additionally, the enclosures for these systems tend to be rather large; on the order of several cubic feet. It should be noted that the vented box systems, as well as higher order band pass designs, exhibit asymptotic free-field low frequency rolloffs greater than 12 dB./octave, which makes them undesirable for this application.
The techniques for low frequency point sources are well known conventional systems include infinite baffle/closed box enclosures, bass reflex/vented box enclosures, and a recent variant of the vented box, the so-called band pass enclosure. All the above enclosures are designed on the premise that the air load is essentially a mass, with a small additional resistive component which actually represents the audible component. Measurement of these systems' far field response has been greatly facilitated by two techniques which do not require the use of anechoic chambers. One technique, utilizes the nearfield pressure response as a predictor of farfield response. The other technique, uses the second derivative of the enclosure's internal pressure, which, as will be demonstrated, has particular significance in the development of the present invention.
The following describes the peculiar behavior of the small acoustic spaces typical of automotive interior listening environments. Acoustic spaces with physical dimensions less than one-eighth wavelength of sound in air can be characterized as a constant acoustic compliance (Cab) (or as the reciprocal property, stiffness): EQU Cab=Vb/ro*c 2 (EQ. 1)
wherein, Vb is the internal volume of air, ro is the density of air (1.18 kg/m 3), and c is the velocity of sound in air (345 m/s).
In order to produce a constant sound pressure (Pb) in an acoustic environment characterized by a constant acoustic compliance we need a sound source whose volume velocity varies inversely with frequency, as shown in the relationship: EQU Pb=Uo/f*2*pi*Cab (EQ. 2)
But the Uo volume velocity of the sound source energizing this constant compliance space is related to outside acoustic pressure P out by the following relationship: EQU Uo=Pout/2*pi*f (EQ. 3)
Combining equations (1) and (2) we see that the sound pressure inside a constant compliance acoustic space varies inversely with the square of frequency for outside sound pressure that crosses the boundary of the constant compliance acoustic space. EQU Pb=Pout/f2*Cab (EQ. 4)
In actual practice, the dimensions of typical passenger compartments approach the 1/8 wavelength requirement over the frequency range of 15 to 45 Hz.
Accordingly, it has been found, based on measurements of the acoustic transfer function (i.e.,sound pressure response/sound pressure driving source) of various vehicular acoustic spaces, that the relationship of driving source pressure to measured acoustic space pressure will vary between a first order, inverse with frequency, and a second order, inverse with squared frequency characteristic. These observations are consistent with conventional in-box acoustic pressure measurements, wherein the internal pressure of an enclosure of small dimensions varies as the partial second integral of the external farfield pressure. In the present invention, the interior of the vehicular cabin is analogous to the interior of the cited enclosure. The variations are due to behavior which is analogous to "compliance shift", which is due to the layer of air in the immediate proximity of the radiating element not exhibiting uniform compression with respect to the rest of the enclosure. This effect will be dependent upon the specific physical configuration of the acoustic space in various vehicles.